There has been much investigation in the development of an ignition apparatus for producing a spark for ignition of an internal combustion engine. As a result, the art has developed a variety of different configurations suited for many different applications. In general, it is known to provide an ignition apparatus that utilizes a high-voltage transformer that includes a magnetically-permeable core and primary and secondary windings. It is typical to use copper wire for the primary and secondary windings.
While there has always been an incentive to reduce the amount of copper wire in an ignition coil (and hence the cost attributable to copper), in recent times, the price of copper has increased over 400%, with the result that the cost of the copper wire in an ignition coil has become a significant portion of the total bill of materials (BOM). A couple of approaches are known in the art that have an effect on the amount of copper wire used in an ignition coil. One approach is to wind the primary winding directly onto a round magnetic core and thus eliminate a primary spool, which reduces the diameter of the primary winding turns, and thus the mean length per turn (MLT). For a comparable number of turns, this approach reduces the amount of copper wire. This approach also reduces the MLT of the secondary winding for the same reason, thereby also reducing the amount of copper wire attributable to the secondary winding. For the first approach, the magnetic core is circular in shape and is typically used with an open magnetic path configuration (i.e., a magnetic circuit with large air gaps).
Another approach is to provide a magnetic core that is rectangular in cross-section, and that is provided generally in a two-piece configuration with either a “C-I” or “E-I” shape. In this second approach, an air gap is provided, but is generally very tightly controlled resulting in a structure with a high magnetic permeability. The rectangular cross-section used in this second approach requires a primary spool for the primary winding and therefore increases the MLT of both the primary and secondary windings. However, the relatively high magnetic permeability of the core structure allows for a reduced number of turns as compared to the first approach. For example, U.S. Pat. No. 6,679,236 entitled “IGNITION SYSTEM HAVING A HIGH RESISTIVITY CORE” issued to Skinner et al. is illustrative of the first approach and discloses a round core with the primary winding wound directly onto the outer surface of the core. As a further example, U.S. Pat. No. 5,285,760 entitled “IGNITION COIL DEVICE FOR AN INTERNAL COMBUSTION ENGINE” issued to Takaishi et al. disclose a C-shaped laminated steel core, illustrative of the second approach described above. Co-pending U.S. patent application Ser. No. 12/325,581 filed Dec. 1, 2008 entitled “IGNITION COIL WITH CYLINDRICAL CORE AND LAMINATED RETURN PATH” addresses some of the problems noted above; however, a C-shaped return path disclosed therein is a stamped part in which scrap is formed during manufacture, and which process itself involves increased cost tooling.
Accordingly, there continues to be a need for an ignition coil that uses a reduced amount of copper wire and/or involves producing less scrap or eliminating costly tooling.